Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Exacting requirements are placed on semiconductor fabrication processes involving ion implantation with respect to the cumulative ion dose implanted into the wafer, implant depth, dose uniformity across the wafer surface, surface damage and undesirable contamination. The implanted dose and depth determine the electrical activity of the implanted region, while dose uniformity is required to ensure that all devices on the semiconductor wafer have operating characteristics within specified limits. Excessive surface damage, particularly chemical etch, or contamination of the surface can destroy previously fabricated structures on the wafer.
In some applications, it is necessary to form shallow junctions in a semiconductor wafer, where the impurity material is confined to a region near the surface of the wafer. In these applications, the high energy acceleration and the related beam forming hardware of conventional ion implanters are unnecessary. Accordingly, it has been proposed to use Plasma Doping (PLAD) systems for forming shallow junctions in semiconductor wafers.
In a PLAD system, a semiconductor wafer is placed on a conductive platen located in a chamber, and the platen functions as a cathode. An ionizable gas containing the desired dopant material is introduced into the chamber, and a high voltage pulse is applied between the platen and an anode (or the chamber walls), causing the formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied voltage causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. A plasma doping system is described in U.S. Pat. No. 5,354,381 issued Oct. 11, 1994 to Sheng.
In the PLAD system described above, the high voltage pulse generates the plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma implantation systems, known as Plasma-Source Ion Implantation, PSII, systems, a separate plasma source is used to provide a continuous plasma. (These implantation systems are also known by several other acronyms, the most common being Plasma-immersion Ion implantation, PIII.) In such systems, the platen and the wafer are immersed in this continuous plasma and at intervals, a high voltage pulse is applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer. An example of such a system is described in U.S. Pat. No. 4,764,394, issued Aug. 16, 1988 to Conrad.
An advantage of a PLAD system over a PSII system is that the plasma is on only when the target object is being implanted. This results in a reduction of chemically active species that are produced by the continuous plasma of the PSII system and hence a reduction in chemical damage to the wafer surface. In addition, the continuous plasma can also cause high levels of implanted contaminants and high levels of particulate formation. The PLAD system improves on the PSII system by turning the plasma off except when the target object is biased to implant ions. This reduces the level of contaminants, particulates and surface etching damage.
PLAD systems have a minimum breakdown voltage V.sub.bd at which the plasma ignites and ions can be implanted. This breakdown voltage V.sub.bd is defined by the physical characteristics of the system, including the cathode surface material, the type of gas present in the system, the pressure P of the gas in the system and the distance d from the cathode to the anode. For a given surface material and gas type, the breakdown voltage curve V.sub.bd is a function of P.times.d and is known as the Paschen curve. The process is well described in plasma physics texts. Typically, the minimum value for the breakdown voltage V.sub.bd is near Pd.apprxeq.500 millitorr-cm. For BF.sub.3, a common feed gas used for PLAD of Si, the minimum breakdown voltage V.sub.bd.apprxeq.530 V. Other dopant feed-gas/substrate combinations will have similar minimum breakdown voltages V.sub.bd. The implant energy of the ions in the plasma is directly proportional to the cathode to anode voltage in prior art PLAD systems.
In PLAD systems, the ion current to the cathode is a function of the applied voltage, gas pressure and surface conditions. For voltages near V.sub.bd, the current is low. As the voltage or the pressure is increased, the current increases. In order to increase current and thereby reduce implant times, it is desirable to operate at higher pressures and voltages above V.sub.bd. Local surface conditions, surface temperature, material, material structure (crystal vs. amorphous), etc., also play a role in the local ion current.
In order to increase production rates, i.e., increase the throughput of target wafers through the doping process, the time it takes to process each wafer needs to be decreased. As is known, the cycle time for a target wafer includes the time needed to evacuate the chamber, the time needed to introduce the dopant feed-gas and bring the chamber to the desired pressure, as well as the duration of time needed to bombard the target wafer with ion current to accomplish the desired level of dopant density. Thus, a way to improve production rates and uniformity using a PLAD system is needed.